Butenes, also called butylenes, are a group of four isomeric hydrocarbons with the common empirical formula C4H8, which have a C—C double bond and thus belong to the alkenes. Due to the C—C double bond, they are important starting substances for chemical syntheses and are required e.g. for the production of butanol, butanone, 1,3-butadiene or also plastics such as butyl rubber. Butene also is educt in the synthesis of methyl-tert-butyl ether (MTBE), which is one of the most important industrially used ethers.
Up to now, most of the C4 olefins are produced by cracking of petroleum, in which long-chain hydrocarbons are split into short-chain hydrocarbons. During cracking, propylene, ethylene and aromatic hydrocarbons also are obtained beside C4 olefins. In the cracking process, the yield of C4 olefins however cannot be increased independent of other products. In addition, C2 or C3 olefins generally are preferred in cracking processes due to their higher market price. However, when the ethylene yield is maximized for example by changing the process conditions in typical cracking processes, the C4 olefin yield decreases accordingly.
To satisfy the worldwide demand of butene, it therefore is necessary to resort to further production processes. Such a production process is the so-called MTP process, in which olefins are produced from methanol (MeOH) by catalytic conversion on a zeolitic catalyst. As is already suggested by the name methanol-to-propylene (MTP) process, the focus of this production process is on the recovery of propylene, but by shifting the process condition, the selectivity of the products obtained can be influenced and the product spectrum thus very well can also be shifted towards butenes.
The fundamentals of an MTP process are described for example in DE 10 2005 048 931 A1, in which olefins are produced from an educt mixture containing steam and oxygenates such as methanol and/or dimethyl ether. By a heterogeneously catalyzed reaction in at least one reactor, the educt mixture is converted to a reaction mixture comprising low-molecular olefins and gasoline hydrocarbons. By a suitable separation concept, higher olefins, above all the C5+ fraction, can at least partly be recirculated into the reactor as recycling stream or for the most part be converted to propylene, whereby the yield of propylene is increased.
The MTP process usually has a propylene yield of about 65% (mole C). Previous MTP processes have in common that by an increased yield of propylene the economy of the process should be improved. DE 10 027 159 A1 for example describes an MTP process with two shaft reactors. For this purpose, methanol vapor is converted to dimethyl ether in a first, heterogeneously catalyzed process step. This dimethyl ether subsequently is split up into two partial streams and supplied to a first and a second shaft reactor, in which a product mixture containing propylene is produced on a zeolitic catalyst. The product stream of the first shaft reactor also is introduced into the second shaft reactor. A comparatively high amount of propylene of up to 50 vol-% thereby is achieved. At the same time, the process is very favorable in economic terms, since the expensive tubular reactors are replaced by comparatively inexpensive shaft reactors.
DE 10 2006 026 103 A1 describes another type of reactor for carrying out an MTP process. Gaseous oxygenates together with steam are converted to olefins at 400 to 470° C. in a closed reactor including several trays. The individual trays are filled with a fixed catalyst bed. Each tray individually is equipped with water and dimethyl ether and/or a liquid phase containing methanol, which is sprayed through several nozzle tubes. Thus, the optimum operating conditions can be set in each tray for a stream with this degree of conversion.
DE 10 2009 031 636 finally describes a process for producing the required oxygenates, in particular methanol and dimethyl ether, which is designed such that it is possible to flexibly switch between a methanol purification and a dimethyl ether production.
All previously known MTP processes have in common that they are optimized with regard to the yield of propylene. C4 olefins, on the other hand, only are obtained as by-product and so far have not been in the focus of the procedure.